The efficacy of Bt cotton plants against field populations of Hencove, :po grinigero has not been consistent over the growing season. A repeatable late-season reduction in efficacy has been observed and variation in efficacy has also been recorded early in the season (Fitt at o1.1998). The early season problems are particularly worrying because the cause is unclear, although hypotheses abound.

The variation in efficacy observed could reflect changes at the level of the genome. The

unexpected inactivation of transgenes leading to the loss of a newly introduced trait has been well documented (reviewed in Stain eta1. , 1997). Transgene silencing in response to environmental stresses, such as high light intensity or elevated temperatures, has been reported both for plants growing in the field and in experimental plants growing in controlled environments (Meyer erg1. , 1992; Walter eta!., 1992). An understanding of the molecular basis for both the developmental(late season) and early season decline in efficacy of Bttransgenic cotton will provide a basis form alding decisions concerning the management of the current Bt cotton lines and the development of new lines with insertions of the Bt transgene.